(1) Field of the Invention
The present invention relates to an admixture for cement compositions, and to cement compositions such as high strength mortar and high strength concrete using the admixture.
(2) Description of the Prior Art
High strength mortar known heretofore is formed by mixing fine aggregate and silica fume in appropriate quantities into a cement paste of about 30% water-cement ratio. Also known is high strength concrete formed by mixing coarse aggregate in an appropriate quantity into such mortar.
The conventional product including silica fume is highly viscous owing to the presence of silica fume. This results in low fluidity of the product filled into a mold, and hence a disadvantage of low workability.
Silica fume comprises ultra fine particles having diameters about 1/100 of cement. When used as it is, silica fume is difficult to handle since it becomes scattered. Therefore, silica fume usually is used in grains. This results in the disadvantage that a required quality cannot be obtained because of poor dispersibility when kneading time is insufficient. To secure high quality, measures such as extension of the kneading time must be taken, which prolongs the construction period.
On the other hand, as disclosed in U.S. Pat. No. 4,793,861, metakaolin including alumina and silica as main components is added as an admixture in order to promote product strength and to increase alkali-resistance of glass fibers contained in a cement-based product.
According to this known technique, metakaolin is obtained by a heat treatment of kaolinite at 700.degree. to 900.degree. C. However, such metakaolin usually is in the form of fine particles having greatly varying diameters, with numerous particles mixed together some of which are below 0.5 .mu.m and others over 8 .mu.m in diameter. The particles below 0.5 .mu.m in diameter have large specific surface areas, and tend to have increased water absorption accordingly. Thus, a large quantity of a water reducing agent must be used in the case of a low water-cement ratio, which results in the disadvantage of high cost. There is a further disadvantage of a prolonged construction period with a long time consumed in the kneading operation as discussed above. Where coarse particles exceeding 8 .mu.m in diameter are mixed, a required quality cannot be secured since concrete is not effectively filled to improve its strength.